In liquid developer systems the liquid developer is generally comprised of a carrier liquid and toner particles in a generally constant ratio. During imaging operations the concentration of toner particles is reduced and concentrated toner is added to return the concentration to its desired value.
It is important that the concentration of particles should be kept within a given range in order to realize consistent copy quality. This requirement is especially important in color printers or copiers, where the quality of the images is especially dependent on the color balance and on its stability.
In general, concentration of toner particles in liquid developers is determined by measuring the attenuation of light passing through a given path filled with the liquid developer. Since the particles absorb and scatter light, the attenuation of the light is related to the concentration of the particles.
U.S. Pat. No. 4,579,253 describes a system in which the beam of light is split into two components only one of which is attenuated by the liquid developer. The concentration is determined from the ratio of the attenuated and unattenuated beams.
Such systems work fairly well in single color systems or in multicolor systems in which there is no cross contamination between the colors. In general, the most troublesome cross-contamination is black toner particles in a relatively low attenuation color such as yellow. Since black has an attenuation several times that of yellow, visually negligible black contamination can effect the determination of the color concentration in a way which seriously disturbs the color balance of the system.
Japanese Patent Publication Kokai 1-148943 describes a system in which the attenuation of beams of light having two different colors are sequentially measured. Using these attenuation values, the publication describes a method for determining the concentration of both the black and the color particles.
Generally speaking, measurement of attenuation caused by toner particles is most effective when a large cross-section is viewed, since this increases the signal level for the brightness of light source. This preferred operation requires a detector with a wide field of view.